ABSTRACT
We determine the mass functions (MFs) and the dynamical parameters of 15 nearby open clusters (OCs), using the unprecedented data set of the Gaia Early Data Release 3. We select the members ...of each cluster by combining the photometric (colour and magnitude) and astrometric (parallax and proper motions) parameters of stars, minimizing the contamination from Galactic field interlopers. By comparing the observed distribution of stars along the cluster main sequence with the best-fitting synthetic population, we find the present-day MF and the binary fraction of the OCs, along with their dynamical parameters like mass, half-mass radius, and half-mass relaxation time. We found that the global present-day MF of OCs are consistent with a single power-law function, F(m) ∝ mα, with slopes −3 < α < −0.6 including both subsolar, 0.2 < m/M⊙ < 1, and supersolar mass regimes. A significant correlation between the MF slope and the ratio of age to half-mass relaxation time is evidenced, similarly to the same conclusion already observed among Galactic globular clusters. However, OCs evolve along different tracks in comparison with the globular clusters, possibly indicating primordial differences in their initial mass function (IMF). The comparison with Monte Carlo simulations suggests that all the analysed OCs could be born with an IMF with slope αIMF < −2.3. We also show that the less evolved OCs have an MF consistent with that of the solar neighbourhood, indicating a possible connection between the dissolution of OCs and the formation of the Galactic disc.
ABSTRACT
We have determined stellar mass functions of 120 Milky Way globular clusters and massive Large Magellanic Cloud/Small Magellanic Cloud star clusters based on a comparison of archival Hubble ...Space Telescope photometry with a large grid of direct N-body simulations. We find a strong correlation of the global mass function slopes of star clusters with both their internal relaxation times and their lifetimes. Once dynamical effects are being accounted for, the mass functions of most star clusters are compatible with an initial mass function described by a broken power-law distribution N(m) ∼ mα with break masses at 0.4 and 1.0 M⊙ and mass function slopes of αLow = −0.3 for stars with masses m < 0.4 M⊙, αHigh = −2.30 for stars with m > 1.0 M⊙, and αMed = −1.65 for intermediate-mass stars. Alternatively, a lognormal mass function with a characteristic mass log MC = −0.36 and width σC = 0.28 for low-mass stars and a power-law mass function for stars with m > 1 M⊙ also fit our data. We do not find a significant environmental dependence of the initial mass function on cluster mass, density, global velocity dispersion, or metallicity. Our results lead to a larger fraction of high-mass stars in globular clusters compared to canonical Kroupa/Chabrier mass functions, increasing the efficiency of self-enrichment in clusters and helping to alleviate the mass budget problem of multiple stellar populations in globular clusters. By comparing our results with direct N-body simulations, we finally find that only simulations in which most black holes are ejected by natal birth kicks correctly reproduce the observed correlations.
Abstract
Dwarf spheroidal galaxies (dSphs) appear to be some of the most dark matter (DM)-dominated objects in the Universe. Their dynamical masses are commonly derived using the kinematics of stars ...under the assumption of equilibrium. However, these objects are satellites of massive galaxies (e.g. the Milky Way) and thus can be influenced by their tidal fields. We investigate the implication of the assumption of equilibrium focusing on the Sculptor dSph by means of ad hoc N-body simulations tuned to reproduce the observed properties of Sculptor following the evolution along some observationally motivated orbits in the Milky Way gravitational field. For this purpose, we used state-of-the-art spectroscopic and photometric samples of Sculptor’s stars. We found that the stellar component of the simulated object is not directly influenced by the tidal field, while ${\approx } 30\!-\!60{{\ \rm per\ cent}}$ of the mass of the more diffuse DM halo is stripped. We conclude that, considering the most recent estimate of the Sculptor proper motion, the system is not affected by the tides and the stellar kinematics represents a robust tracer of the internal dynamics. In the simulations that match the observed properties of Sculptor, the present-day dark-to-luminous mass ratio is ≈6 within the stellar half-light radius (≈0.3 kpc) and >50 within the maximum radius of the analysed data set (≈1.5○, ≈2 kpc).
ABSTRACT
We present the results for a sample of B stars in the Large Magellanic Cloud young double stellar cluster NGC 1850 A and NGC 1850 B, as observed with the integral-field spectrograph at the ...Very Large Telescope, the Multi Unit Spectroscopic Explorer (MUSE). We compare the observed equivalent widths (EWs) of four He lines (4922, 5015, 6678, and 7065 Å) with those determined from synthetic spectra computed with different He mass fractions (Y = 0.25, 0.27, 0.30, and 0.35) with the code synspec, which takes into account the non-local thermodynamic equilibrium effect. From this comparison, we determine the He mass fraction of the B stars, finding a distribution that is not homogeneous. The stars can be divided in three groups: He-weak (Y < 0.24) and He-normal (0.24 ≤ Y ≤ 0.26) stars, belonging to the main sequence of NGC 1850 A, and He-rich stars (0.33 ≤ Y ≤ 0.38), situated in the main sequence associated with NGC 1850 B. We analyse the stellar rotation as possibly being responsible for the anomalous features of the He lines in the He-rich stars. We provide a simple analysis of the differences between the observed EWs and those obtained from theoretical models with different rotation velocities (Vsini = 0 and 250 km s–1). The resolution of the MUSE spectra does not allow us to obtain a conclusive result; however, our analysis support the He-enhanced hypothesis.
ABSTRACT
We present the result of a survey of Monte Carlo simulations of globular clusters hosting two generations of stars including a large ($f_{b}=50{{\ \rm per\ cent}}$) fraction of primordial ...binaries in both populations. The dynamical evolution of the two stellar populations is followed for a Hubble time taking into account the effect of the tidal field, two-body relaxation, stellar evolution, and three/four-body interactions. The fraction of surviving binaries, once accounted for the observational bias and uncertainties, is compared with the available radial velocity time-series performed in real globular clusters, and it is used to constrain the initial spatial concentration of the second generation. The fraction of second generation binaries appears to depend only on the ratio between the total cluster mass and the initial size of the second generation that determines the average velocity dispersion across the extent of this stellar population. In spite of the various uncertainties, we find that the observed fraction can be obtained only assuming a strong initial concentration of the second generation ($r_{h,S}\sim 0.1~(M/10^{6} \, \mathrm{M}_{\odot })\, \mathrm{pc}$). The evolution of the first generation binary fraction is more sensitive to the tidal field strength (with a non-negligible effect of the cluster orbital eccentricity) since the tidal field has a direct impact on the first generation structural properties.
Abstract
We present the chemical analysis of 49 giant stars of the globular cluster NGC 2419, using medium resolution spectra collected with the multi-object spectrograph DEIMOS@Keck. Previous ...analysis of this cluster revealed a large dispersion in the line strength of the infrared Ca ii triplet, suggesting an intrinsic star-to-star scatter in its Fe or Ca content. From our analysis, we assess that all the investigated stars share the same Fe/H, Ca/Fe and Ti/Fe abundance ratios, while a large spread in Mg and K abundances is detected. The distribution of Mg/Fe is bimodal, with ∼40 per cent of the observed targets having subsolar Mg/Fe, down to Mg/Fe ∼ −1 dex, a level of Mg deficiency never observed before in globular clusters. It is found that the large dispersion in Mg abundances is likely the main origin of the observed dispersion of the Ca ii triplet lines strengths (that can be erroneously interpreted in terms of Fe or Ca abundance scatter) because Mg plays a relevant role in the atmosphere of giant stars as an electron donor. A strong depletion in the Mg abundance leads to an increase of the line strength of the Ca ii triplet, due to the variation in the electronic pressure, at a constant Fe and Ca abundance. Finally, we detect an anti-correlation between Mg and K abundances, not easily explainable within the framework of the current nucleosynthesis models.
ABSTRACT
We present the results of the analysis of deep photometric data of 32 Galactic globular clusters. We analysed 69 parallel field images observed with the Wide Field Channel of the Advanced ...Camera for Surveys of the Hubble Space Telescope which complemented the already available photometry from the globular cluster treasury project covering the central regions of these clusters. This unprecedented data set has been used to calculate the relative fraction of stars at different masses (i.e. the present-day mass function) in these clusters by comparing the observed distribution of stars along the cluster main sequence and across the analysed field of view with the prediction of multimass dynamical models. For a subsample of 31 clusters, we were able to obtain also the half-mass radii, mass-to-light ratios, and the mass fraction of dark remnants using available radial velocity information. We found that the majority of globular clusters have single power-law mass functions F(m) ∝ mα with slopes α > −1 in the mass range 0.2 < m/M⊙ < 0.8. By exploring the correlations between the structural/dynamical and orbital parameters, we confirm the tight anticorrelation between the mass function slopes and the half-mass relaxation times already reported in previous works, and possible second-order dependence on the cluster metallicity. This might indicate the relative importance of both initial conditions and evolutionary effects on the present-day shape of the mass function.
Abstract
Using data from Gaia DR2, we study the radial number density profiles of the Galactic globular cluster sample. Proper motions are used for accurate membership selection, especially crucial ...in the cluster outskirts. Due to the severe crowding in the centres, the Gaia data are supplemented by literature data from HST and surface brightness measurements, where available. This results in 81 clusters with a complete density profile covering the full tidal radius (and beyond) for each cluster. We model the density profiles using a set of single-mass models ranging from King and Wilson models to generalized lowered isothermal limepy models and the recently introduced spes models, which allow for the inclusion of potential escapers. We find that both King and Wilson models are too simple to fully reproduce the density profiles, with King (Wilson) models on average underestimating (overestimating) the radial extent of the clusters. The truncation radii derived from the limepy models are similar to estimates for the Jacobi radii based on the cluster masses and their orbits. We show clear correlations between structural and environmental parameters, as a function of Galactocentric radius and integrated luminosity. Notably, the recovered fraction of potential escapers correlates with cluster pericentre radius, luminosity, and cluster concentration. The ratio of half mass over Jacobi radius also correlates with both truncation parameter and PE fraction, showing the effect of Roche lobe filling.
ABSTRACT
We developed self-consistent dynamical models of stellar systems in the framework of quasi-linear modified Newtonian dynamics (QUMOND). The models are constructed from the anisotropic ...distribution function of Gunn and Griffin, combined with the modified Poisson equation defining this gravitation theory and take into account the external field effect. We have used these models, and their Newtonian analogues, to fit the projected density and the velocity dispersion profiles of a sample of 18 Galactic globular clusters, using the most updated data sets of radial velocities and Gaia proper motions. We have thus obtained, for each cluster, estimates of the dynamical mass-to-light ratio (M/L) for each theory of gravity. The selected clusters have accurate proper motions and a well-sampled mass function down to the very low-mass regime. This allows us to constrain the degree of anisotropy and to provide, from comparison with stellar evolution isochrones, a dynamics-independent estimate of the minimum mass-to-light ratio (M/L)min. Comparing the best-fitting dynamical M/L with (M/L)min, we find that for none of the analysed clusters the two gravity theories are significantly incompatible with the observational data, although for one of them (NGC 5024) the dynamical M/L predicted by QUMOND lies at 2.8σ below (M/L)min. Though the proposed approach suffers from some limitations (in particular the lack of a treatment of mass segregation), the obtained results suggest that the kinematics of globular clusters in a relatively weak external field can be a powerful tool to prove alternative theories of gravitation.
Abstract
We present a detailed analysis of the kinematic properties of the multiple populations (MPs) in the low-mass Galactic globular cluster (GC) NGC 6362 based on a sample of about 500 member ...stars for which radial velocities (RVs), and Fe and Na abundances have been homogeneously derived. At distances from the cluster center larger than about 0.5
r
h
, we find that first-generation (FG–Na-poor) and second-generation (SG–Na-rich) stars show hints of different line-of-sight velocity dispersion profiles, with FG stars being dynamically hotter. This is the first time that differences in the velocity dispersion of MPs are detected using only RVs. While kinematic differences between MPs in GCs are usually described in terms of anisotropy differences driven by the different radial distributions, this explanation hardly seems viable for NGC 6362, where SG and FG stars are spatially mixed. We demonstrate that the observed difference in the velocity dispersion profiles can be accounted for by the effect of binary stars. In fact, thanks to our multi-epoch RV measurements, we find that the binary fraction is significantly larger in the FG sample (
f
∼ 14%) than in the SG population (
f
< 1%), and we show that such a difference can inflate the velocity dispersion of FG with respect to SG by the observed amount in the relevant radial range. Our results nicely match the predictions of state-of-the art
N
-body simulations of the co-evolution of MPs in GCs that include the effects of binaries.
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